The latent emission potential of formaldehyde from urea-formaldehyde resin bonded lignocellulosic material, such as particle board and plywood, has greatly restricted the use of such composite materials in applications in indoor environments. The restriction is due to the fact that formaldehyde is considered a carcinogenic agent and is an irritant when present even in parts per million in a closed environment.
The reason for the slow liberation of formaldehyde from binder/lignocellulosic systems has been the object of extensive research for many years. It is well known that UF resins with a high mole ratio of formaldehyde to urea have specific chemical bonds between the cellulosic material and methylol or formaldehyde ureas that undergo chemical degradation due to atmospheric moisture, changes in pH, temperature and other factors which play an important role in the formaldehyde emissions.
In order to overcome the above-mentioned problems, various methods have been used in recent years. One method is to reduce the formaldehyde:urea ratio in the resin. A second method is to use formaldehyde scavengers during application of the resin. A third method refers to the use of latent reactants. Furthermore, the use of less volatile aldehydes in place of formaldehyde helps to solve emission problems. However, almost all viable substitutes for formaldehyde in lignocellulosic composite materials are too expensive.
Another problem appears when the formaldehyde:urea mole ratios are lowered. The internal bond strength of the material decreases. This requires the use of a greater amount of resin which renders the process unattractive.
The use of formaldehyde scavengers results in a low formaldehyde emission during applications but does not prevent the slow and continuous liberation of formaldehyde subsequent to the application.
Thermodynamic considerations on the methylolation of urea, which is the initial reaction between urea and formaldehyde, lead to the conclusion that the third and fourth (theoretical) positions of the methylol groups in the four possible methylol urea positions are weakly attached and hence are potentially more labile, i.e., subject to relatively easy degradation. It is well recognized that the methylol ureas are responsible for the gluing characteristics of the resin to a cellulosic material and are involved in chemical reactions with the substrates. Thus, if a part of these labile linkages can be substituted by less volatile aldehydes compared to formaldehyde, the liberation of formaldehyde due to chemical degradation decreases.
The present invention is based on the above considerations and refers to a binder system with improved adhesion properties and low formaldehyde liberation in order to meet the DIN E-1 specifications, that emission of formaldehyde by the perforator test must be less than 10 mg/100 g.
The present invention discloses a process for the production of a binder with low emission of formaldehyde, a composition comprising said binder which is used in the production of particle board and plywood and a process for crosslinking said composition when added to these lignocellulosic materials.
In the process of this invention part of the formaldehyde in the urea-formaldehyde binder is replaced by a higher aldehyde which is generated in situ from commonly available carbohydrates such as starch, dextrose, saccharose, etc. The use of starch, dextrose, and saccharose in urea-formaldehyde polymer systems has been reported previously. These carbohydrates have been used to reduce polymer costs, to increase plasticity of the cured polymer, or to increase viscosity or tackiness of the polymer, etc. However, such uses have not comprised preparation of aldehydes from carbohydrate systems in order to copolymerize them with the urea-formaldehyde resin and produce a polymer system wherein the post-emission of formaldehyde is greatly reduced.
The present invention is specifically directed to lignocellulosic/polymer composites, wherein the polymer is the binding agent, and intends to solve the problems of the state of art, by reducing formaldehyde emission while maintaining the adhesion characteristics of the polymer.